COMPLEMENT, that which fills up or completes anything, e.g., the number of men necessary to man a ship (Lat. complere, to fill up). In geometry (q.v.) the complement of an angle is the difference between the angle and a right angle; the complements of a parallelogram are formed by drawing parallel to adjacent sides of a parallelogram two lines intersecting on a diagonal; four parallelograms are thus formed, and the two not about the diagonal of the original parallelogram are the complements of the parallelogram. In analysis, a complementary function is a partial solution to a differential equation (q.v.) ; complementary opera tors are reciprocal or inverse operators. A "complementary colour" is one which produces white when mixed with another. (See COLOUR.) In Spanish the word cumplimento was used for the performance of acts of courtesy, and it came into use in English, with a change in spelling to "compliment," with the sense of an act of politeness, especially of a polite expression of praise.